A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to displace and position the substrate and/or the patterning device, the lithographic apparatus usually includes a plurality of actuators and/or linear motors. In general, such motors or actuators include a magnet assembly and a coil assembly whereby an interaction between both enables a force to be exerted on an object connected to either the magnet assembly or the coil assembly. In order to improve the efficiency of such motors or actuators, different alternatives for the magnet assembly are disclosed in the art.
One way of improving the efficiency is the introduction of a so-called Halbach configuration of magnets. Compared to a conventional array of permanent magnets arranged to generate an alternating North-South pattern, a Halbach configuration of magnets introduces additional magnets arranged between the alternating North-South pattern. As a result, the magnetic field generated by the magnet assembly can be increased in the area where the coil assembly is arranged. A drawback of the Halbach configuration is that the additional magnets introduce additional costs and additional time to manufacture the magnet assembly. Furthermore, a risk of demagnetization part of the permanent magnets may occur due the introduction of the additional magnets.